This invention relates to counterbalance-type heavy lift cranes for use in the construction industry, and more particularly to a pivotally-mounted, reeved counterweight system connected to the rearward end of the crane deck or platform of a mobile or fixed counterbalance type crane assembly.
Most construction companies will have one or more general, all-purpose truck crane or crawler cranes which are used in the majority of lifting and moving projects encountered in a typical construction project. Such cranes currently have reached the size and weight limitations for ease of transport among the various job sites on public roads while the lifting capacity requirements of the construction industry have continued to increase.
Designers of portable lift cranes have been faced with the problem of creating a portable crane that is readily transportable among the various job sites within existing highway size and weight regulations while increasing the lift capacity of the crane. There have been various methods of making the counterweights both rearwardly extendible from the crane platform or deck as well as removable from either the platform or from their connection from behind the crane and platform.
Removable counterweight and counterweights detachable from the crane for transportation by another vehicle have been built and are known. The problem with removable counterweights is that they require perhaps another crane to place and remove the counterweight between the crane and a transport vehicle. Other removable counterweight systems require substantial disassembly to reduce their weight for transportation. Wheeled counterweights that rigidly and rearwardly extend from a counterweight arm on the platform are plagued with the problem of jerking the boom of the crane, as they move or rotate simultaneously. Also, such wheeled counterweights do not provide adequate counterweight for most cranes. Most of their counterweight force is borne by the wheels on the ground and not by the counterweight arm or boom thereby decreasing the lift capacity of the crane while still problematically increasing the total mass of the crane assembly.
Another problem with counterweights that extend from the counterweight arm or the deck of the crane is structural stress upon the deck or counterweight arm and the rotational pivot point of the deck. This stress is caused by having the counterweight located substantially rearward of the rotation point of the deck and having such counterweights substantially supported by the deck or counterweight arm which causes structural fatigue or buckling of portions of the deck or platform.
Apart from counterweighting the boom, crane backward stability is also important. The crane industry is looking for a simple and lower cost method of counterweighting a crane to prevent forward tipping where the problem of backward stability is also solved. In the past, backward crane stability, which must be considered in designing a crane, required extensive assembly time with costly components. One such example is the AMERICAN SKY HORSE.RTM. made by the American Hoist & Derrick Company. The SKY HORSE.RTM. sells for around $400,000.
There is a need for a counterweight system for a counterbalance-type crane with a counterweight arm extending rearwardly from the crane's rotatable deck or platform. Such a system suitably would include a pivotally-mounted wheeled counterweight carriage that would not put excessive stress forces or fatigue on the counterweight arm or crane platform. Such a system should greatly increase the crane's lifting capacity limited only by the structural limitations of the other elements of the crane assembly such as the ropes, pendants, and perhaps the boom itself. The system should assemble quickly and cost under $100,000.